The present invention is a method and apparatus that provides a system for calibrating the ion current projected past each modulation electrode of an ionographic printing head to achieve a uniform value of current past each electrode. Uniformity of ion current under conditions of a known modulation voltage is important in producing high quality printed images, particularly those in which a grey scale is desired. Non-uniform densities of ion current flowing past each modulation electrode often result in unacceptable variations in printed image density.
Ionographic printing is that type in which charged particles are created at a remote location from the point at which a charge latent image is formed on a dielectric surface of a conductive substrate receptor. A stream of ions, assisted in some known systems by a fluid stream, generally an air stream, passes through a channel in the print head in which is placed a linear array of modulation electrode fingers, one for each pixel in a line of the printed image. Ions are created in a chamber, for example, by electrical discharge from a corona wire, maintained at very high positive voltage. During the printing process, the stream of ions is projected toward the receptor, usually a drum, which is placed at a high, negative electrical potential so as to attract the ions to its dielectric surface. Those ions which are projected from the printing head are pulled to the dielectric, over-coated surface of the receptor where they form a charge image, ready for immediate Xerographic-type development into a printed image. Application of low, modulation voltages to the electrode fingers can deflect the ions and locally eliminate them from the stream, producing varying densitites of ions and thus varying density of the subsequently printed image.
In high-quality ionographic printing, particularly where grey-levels are used, unless non-uniformities of each of the ion steams are corrected, unacceptable variations in the printed image density occur. Prior art has addressed the problem of controlling ion perturbation at the imaging surface caused by the effect of previously deposited ions on the path of subsequent ions directed toward the imaging surface. The present invention is directed to compensating for dimensional and electrical variations in the ionographic printing head which cause non uniformity of individual ion streams across the width of the printing head. As used herein, "calibrating" is the process of making the ion stream flowing past each modulation electrode uniform for a desired value of modulation voltage. In calibrating the output of an ionographic head, it is desirable to calibrate at one time as many modulation electrodes as possible, because of the large number involved. For example, a printing head for an eight and one-half inch wide page, at a resolution of 300 spots per inch, might contain 2,560 modulation electrodes.
The problems of eliminating mechanical scanning and of rapidly calibrating a large array of modulation electrodes in an ionographic head have been major ones for designers of ionographic printing devices. The development of a direct way to insure uniform ion flow in spite of variations in the construction of ionographic printing heads would be a major technological advance and fill a long felt need in electronic printing.